Apparatus for grinding drills



June 27, 1950 M. DOUGLAS APPARATUS FOR GRINDING DRILLS Original Filed001,- 18, 1945 3 Sheets-Sheet l INVENTOR.

elvjlz flou glag June 27, 1950 M. DOUGLAS APPARATUS FOR GRINDING DRILLS3 Sheets-Sheet 2 Original Filed Oct. 18, 1943 AX/s PL AME A 6 ONE AX/5 6DIP/1A AXIS .n 1 7 F m0 June 27, 1950 M. DOUGLAS 2,512,838

APPARATUS FOR GRINDING DRILLS Original Filed 001;. 18, 1943 5Sheets-Sheet 3 INVENTOR.

Melt 1'11 flolfglds BY MW% Attys Patented June 27, 1950 2,512,888APPARATUS FOR GRINDING DRIIZLS Melvin Douglas,'Chicag0,'IIll., assignor'to Henry Hildebrandt Continuation of application .-Serial No. 2506;695,.October 18, .1943. This application May-8,.1948,

.Serial No. 25,885

' 2 Claims.

This application is a continuation of my copending application Ser;No.506;695,"filed*October 18,1943, now abandoned.

The ordinary twist .drill-which is 'nearly always used in drilling holesinrmetaland is commonly1used for drilling holes in'manyother'materialsmust be ,ground very precisely to function at "its best. Thehandvgrinding'or sharpening of these drills requires a very high degree "ofskill. "Not only 'are"'the'manipulations of the drill against the'grindstone J complex, but. the 3 operator must alsol'be able toexercise his judgment accurately as "to' the desired .angles .andvariations thereof and theiuniformity of the sides. 'Verycomplex drill.grinding machinery has been developed and used extensively. Not onlyare these machines extremely :expensive, but they require skilledoperators.

The problem of "drillgrindingis complicatedby the desirability ofi-ncreasing--the-- clearance behind the cutting edge from the peripheryof the drill towards its axis. The end :or :point surfaces of. thedrilhthose tworsurfaces at thezen'd of :the helical lands-arecalled theheels. :The ordinarydrill has two lands separated :.by two flutes andthencetwo heels. Thefront-edge :of each heel forms a cutting edge'orlip'of the'dr ill. It is'obviou'sly necessaryfor the heel -to be groundoif enough toprovide a clearance angle behind the cutting edge-sothatthe heel-will not'ri deon thesurface ofthe m'etalat the bottomof"the' ho'le being drilled and prevent the cutting.- edge frombitingintothe metal. If the clearance-ang le i-s too great,the drill isexcessively weakened and the cutting edge will be chipped off'finser'vice.

- It follows that although 'itis necessary-taprovide'sufficientclearance to" allow the cutting edge to bite into the metal,theclearance angles-should not-greatly exceed this" requirement '--orthe drill Will be weakened without any corresponding'benefit.

The theoretical clearance angle is considerably greater '-near the axisof thedr'ill than nearits periphery. The reason for this will beapparent by contemplating the -path of movement of two points of thecutting edge ofthe drill, one'at'the periphery and one" adjacent thecentral web *of the drill. For example, let us-assume-a one inch drill"and consider "a "point at the-periphery and a point .1 inch "fromthe"axisof the drill. L If the drill is -fed-one ="thousandth of an inchperrevolution;both points moveinto'themetal 'beingdrilled one=thousandth"of an inch feach" revolution but the peripheral point, "in advancingthis tonethousandth of an inchrmovesa 'peripheralfldistance equal to theicircumference 'of"a"cir.cle of one inchtdiameter;'namely"3l4l6 inches.The point v.1 ,inch'from'ithe axis,on"the other hand, hasaperipheral'movement .equal"to* the-"circumference of a circle f 1:2diameter,"namely .6283 while "advancing :its '.00 '1inch. Thus itfisseen that thelinner'po'int advances'xdtimes. as steeply into -the'metal"as the outer point and 'hence" it wouldseem that it must have. 5 timesthewlearance angle behind. itin orderthatthe movement of thecuttingedgecan bite'freelyi into the metal. However, the metaladjacentthecutting edge is naturally subject to some elasticdistortionmnder thepressures applied to theman'cl thisdistortion may befairly uniform' along "the entire :cutting edge. Asa result-ofallowingfor :t'his'elasticity, the clearance angle-required at the-innerpoint is in "fact not. nearly 5 times 'asiigreatias' at the peripheryand'a; great dealmore clearance must be allowed at 'bothpointsthan'WOLlldJTbB indicated by the. gradual. rates at whicnthey move into the"work. Nevertheless"'the "fact remains' that a' considerablylargerclearanceari'gle is desirednear .the center voiithe drillthannearits' periphery.

This complicated shape has been obtained heretofore byconsiderable:manipulation of the drill on the grindstone, thismanipulation being either manualor by more or" less automatic machinery.The more complex shape described-has been obtained in' a-variety ofways, one of-which has involved moving the drill across an-edge of a'grindstone so that it was difierentially *ground while-rotating thedrill-atthesame time. of course," only'took'careof 'one' land of the-"drill and "it-was necessary'to go through"the-whole complicatedmovement-with the drill shifted I degrees between-times in order togrind the other land to match.

According to the present invention,-this"complicated movement of the"drill which requires either special skill-or special machinery or bothiSaVoide'd. The "drillmay-simply be pressed-with little or'norotation-against -a special *grindstone, withdrawn, rota'tedIS'Odegrees about its "axis and again pressed without rotation against thegrindstone. The shape of the heel surface is determined accurately,uniformly and without the exercise of any operating skill by thepredetermined shape of the grindstone and the predetermined positioningof the drill. In other words, the drill is contour ground. Prior to thisinvention, the provision of the complex shape heretofore describedwithout manipulation of the drill would have been considered impossible.It is accomplished according to the present invention simply by pressingthe heel surface to be ground against an inside conicalsurface of aspecially contrived grindstone.

An unskilled operator could be trained quickly to do a fairly good jobof drill sharpening by" pressing the drill against this grindstone byhand. 3

However, the extremely simple movement of the drill permits theprovision of a very simple device for holding the drill in the properposition and shoving it against the grindstone with greater ac- 4degrees and pressed to the same axial position while the other heel isbeing ground. This ensures uniformity between the two heels and thepositioning of the drill on the internal conical surface of the stoneensures proper shaping of each heel. Proper positioning of the drill inthe two positions is ensured by a holding and manipulating unit 2! whichcan be much more simple than any prior drill sharpening apparatusbecause of the simple movements of the drill which are sufficient foraccurate grinding, according to the present invention.

The drill is accurately positionedin the unit 2| by placing it against aguide 22 which is swung into position as shown in dotted outline in Fig.2.

"During theinsertion of the drill, the handle 23 (which moves chuck 24in cradle ring or carriage would be held by lock 28 in the positionshown curacy and uniformity than would be possible in holding the drillby hand. This holder has been worked out to be adapted to a largevariety of sizes of drill even though drills of different sizes shouldbe centered at different positions with respect to the grindstone.Adjustment is also provided for varyingfthe clearance angles accordingtothe work to beperformed.

Additional objects and advantages of theinventio'n'will be apparent fromthe following description and from the drawings in which Fig.1 is afragmentary'perspective view of the principal portions of one embodimentof the invention chosen for illustration Fig. 2 is a perspective view ofthe drill holding apparatus shown in Fig. 1, Fig.2 showing the position.of the parts in setting the drill in the chuckf Fig. 3 is a view'illustrating the proper drilling action of a correctly ground drill;Fig. 4 is a 'sideview of the en'd'portion' of a drill; Fig. 5 is a Viewvof the end of a drill; U FigIi'G is a view" illustrating (one possiblepositioning ofIthe drill against'the internal conical surface of thegrindstone; b

Fig. 7 7'is a plan view of the embodiment of this invention chosen forillustration; 'Fig. 8 is aside elevational' view of the same;

Fig. 8'A is a. fragmentarydetail viewof the In accordance with thisinvention, a special grindstone ll isprovidedhaving an internal conicalsurface 12. This grindstone is carried; and driven by the shaft of. amotor I3. Adrill I4 is sharpened by simply pressing the successivesurfaces tobe ground against theinternal conical surfacelZ. Ofcourseproper positioning of the drill is important. In the ordinary drill,there are two surfaces tobe ground,;these surfaces being the ends of thetwo lands "land l1. These end surfaces are called the heels. One of themis seen in Figr i andis numbered 18.

It is, of course; important that the two heels be ground evenly as wellas that each have the proper. shape. According to this inventioir thedrill is pressed to aI-given position while one heel i P ge oun thenbest d r t ted 1 0 in Fig. 1. The chuck jaws 26 are tightened on V thedrill while the parts are thus disposed.

The guide 22 is swung out of the way, the handle 23 is moved to theintermediate position shown in Fig. 2, and the entire unit 2| is slidalong base 21 (Fig. 8) into operating position, predetermined asdescribed below. The handle 23 is then swung to the rightor left.Approximatel'y'at the end of1fthe;slot'2j 9,'the slot curves forwardlyas shown at'fifl' soIthat moving the handle as far as it will gate theleft also pushes it and chuck 24 forwardly. Thehandle Z3. is rigidlymounted in chuck ,24 which is rotatable and longitudinally movable inring or carriage 25. Thus as the handle, is'swung to the. left, andpushed forwardly by 'thecu'rve 30 in slot 29, this presses the drilllll'again'st the internal conical surface of grindstone l2,,thu's grinding.oneheel of the'drill. The handle 23'. is then swung all the way to theright where the other end of the slot is also shaped as shown at 530.This rotates the drill ldegreessojasto' present the other heel to theinternal conical surface I2 of the stone and topress' it against thestone to the same axial position as before so that the other heel of thedrill is ground to-match the first heel.

Theory] of drill grinding I The proper grinding ,of a 'drill is perhapsmost evident in the larger sizes where a properly ground drill willcut-twouniform'coiled shavings 36 from the'metal 31 into which it isbeing uniformly fed.-These coiled shavings (or chips, as they arevcalled in the art) are cut-from the metal bytwo cutting edges 38 formingthe front edges of the heels l8; v; In order for the cutting edges tobite into themctal'being drilled,- it is, of course, necessary that theremainder of the ,heel behind the cutting edge'have-sufiicientclearancewith the metal at the ,bottom'ofthe hole so that the heelsurface will not rest on the bottom of the hole and keep the cuttingedge fromgoing deeper Since the drill is being constantly advanced bythe feed thereof, the cuttingedges are constantly moving moredeeply-into the metal with a result that the surface .atthe bottom ofthe hole is of generallyvconvolute shape. If theheelsurfaces ofthe'drill had the same convolute shape as the bottom'ofthe drill hole,they would rest with equal pressure ;at allpointsr -Of course it isdesired to concentrate z-the pressure at the cutting edge andhence;theheel-should have a greater pitch or clearanceangle "than thepitch of the convolute formedby the -bottom; of the hole.

If we referto Fig 4 'enlightening comparisons may bemade between thenovements of the rqi i' smi t e Po ts: and; etween the clean ancez anglerequired :behindeach. Let 11s assume that the point A is at theperiphery of thesdrill andlxthe point B. Lisnear. thenaxis-say.onefi'fth of;the :distance outtoathe periphery. .'=As:the drill fed,both: :poir1ts:move .into the, metal and the sameadistance ofor eachrevolutioni'of the'drill. 51h aperipheral direction,.howeVer,-1thepointA moves five ::times :the. distancesthat the .point' B moves. Itfollows that since,the;point='Amovesjfivetimes the distance that thepoint B,moves for the same advance into themetalfthe point B has a pitchor slope of movement-five times that ,of point A. -Likewise the slope.of the helix described by point-Balong the bottom surface of the. holeis five times as steep as the helix described by point, A. ..It followsthat, ignoringdistortion of themetal, the clearance angleof the heel I8must befive times as; steep behind the point B as behind the point .A..In fact, the resilient distortion cannot be ignored and hence theclearance. angle ..does .notneed to increase inwardly nearly as rapidlyas. would be indicated byv these ratios. Nevertheless, .agreaterclearance ,angle is required .near theaxisof the drill than at theperiphery.

.It. isv not .practicable merelyto provide along the entire heel surface18 themaximum angle of. clearance that is required because this wouldunnecessarily. weaken thecutting edge and cause it to chip off .inservice.

It is evident from the foregoing that there are important requirementsas to the amount of clearance angle and astothe increasing of theclearance angle inwardly from theperiphery of the drill. This 'does notmean, however, that there can be no "departures from one theoreticalshape. '.The shape which with the vmaterial being drilled would justbarely provide clearance above the bottom of the hole mighttheoretically be the best. shape but in fact there are probably noobjections to'any variations between this. shape andthe higher clearanceanglesat which chip: ping of the cutting edge begins to occur.Furthermore, there is considerably more leeway quite close to the axiswhere the areas and torque arms involved are small and where thereisrelatively little metal that has to be removed in drilling the hole.The clearanceangle desired varies according to the material to bedrilled, the rate of feed and perhaps other considerations. The drillshown in Fig. 4 illustrates a very common clearance in'which a linefromthe point A to thepoint where the land joins the heel and the flute atthe edge opposite theflip forms a 12 degree angle with a planeperpendicular to the axis of the drill. This. is generally referred toas a 12 degree clearance although in fact, the clearancepimmediatelyadjacent to the point A is; much less than 12 degrees. For the. purposeof comparison, the clearance angle at the point B hasbeen indicated asbeing 23 degrees. Again, the clearance'angle immediately behind point Bmay be "lessthan the 23. degrees there indicated. These clearanceangles, as commonly referred to, mean the apparent overall angle.However, a more common-way of judging the clearance at themore-central-portions of the drill can be seen with-reference to Fig. 5.Generally speaking, the angle between the cuttingedge 38 and theconnectingedge 39 crossing the dead center of the drill increases as theclearance near the axis increases. It is-common 'to' provide clearanceat the central portion of the drill such that this angle between edges38 and 39 is from 120 to 135 degrees. I

.The...angle ofauthe :cutting zedge with the axis of 6. the -Idril1 isalso important, although .-:the..59:Lde gree 'anglecillustratedin 'Eig;6 is very .much more common. than any other angle. Naturally it' is alsoimportant to have. the two cutting. edges identical or one will do more.work than the other, .or the drill .mayxdrill off eenter .and.drillahole that .is i.-too' :large or. irregularly shaped.

-Grindiugon the conical stone I'Ifheshapeoflthe drill surfaces 'in' thevarious respects.discussedabove can be predetermined inaccordancewiththe present. invention by selectinga stonehaving a suitableangularity of its cone. and byproperly selecting. the position offthedrill as it is pressed against'the internal conical grinding surface. Itis not meant to imply that the conical grinding surface will shape theheel to' an accurate convolute. It may, however, come closer toproviding-a theoretically perfect surface than any previousxcomparablemachine and closer than is achieved by the. most skilledhand-sharpening, except in rare instances of good'luck. At least testshave shown that it comes withinthe permissible limits. 'of departurefrom a theoretically perfect surface.

Any internal grinding surface may be used to obtain the advantages ofthis invention to some extent. In'fact,'if point shapes not nowcommonwere desired, .any internal grinding surfaces of progressivelydecreasing radius would bewithin the range of'this invention. andsimplicity. of this invention may possibly re sult in the production ofdrill shapes even more desirable than present common drill shapes. and,of course, the inventioncan be used to produce a wide variety of shapeswhether better than common drill'shapes or not. It is preferred,however, at least for the present, that a'surface be chosen, and apositioning-or the drill thereon, such as. will approximately reproduceconditions now more or less regarded as standard.

For the convenience of those "wishing "to practice the invention, onecone size anddrill positioning thereon that "have been foundsatisfactory will be described, but it should-be recognized that this isjust one of theman'y ways in which the resulting shape can be producedor closely approximated.

One cone size which has been "found very suitable is cone having avertex angle of -82 degrees. In other words, an element of the cone (aline extending from the vertex along the cone surface in an axial plane)makes an-angle with the axis of ll'degrees.

Although'cones of almost any angle can pro- Vide some of the advantagesof this invention, those between approximtely 59 and approximatelyllS"have some advantage for drills having lip-angles of 59. A smaller anglethan 59 at the vertex'will lcause the body of the drill to strike thecome at its eriphery in moving the. drill point close to the vertex. Alarger angle than 118 may not closely approximate the conventionalsurfaces.

The proper positioning of the drill on such a surface will dependsomewhat'on the size of the drill. It has been determined, however, thatwhen only one type of point is desired, good results can be obtained-bykeeping all adjustments constant except the distance ofthe drill fromthe axis of the cone. Hence the only variation in positioning that needsto --be -made for difierent sizes of drills is to move the dead'centerof "the drill along an element of the coneand hence the chuck may bemounted to move m The accuracy a direction parallel with the coneelement .on' which the drill is centered, and .no..other adjustmentwillordinarily be necessary. This cone element of the cone maybe referred toas the chosen element.

With the conventional surfaceinwhich the lip has an apparent angle ofapproximately 59 with the axis of the drill, the lip can be placedapproximately parallel to the chosen element of the grinding cone andthe drill positioned with its axis forming an angle of 59 with saidelement- The lip is then bound to have the desired angularity with theaxis ofthe drill. Speaking more accurately, it should be noted that aline or element of the heel surface extending from the dead center andwhich lies against the chosen element of the cone, is bound to make anangle with conventional practice.

The angle of clearance is determined in part by the angle which the axisof the drill makes with that plane which includes both the axis of thedrill and the chosen element; it being assumed again that the lip of thedrill is approxi mately in this plane. In Fig. 6, this'plane'has beenidentified as axial plane A and its intersections with the cone havebeen shown. The drillaxis has been shown in Fig. 6 intersecting thechosen element 4| at a point'C. This is the point at which the deadcenter of the drill is placed. One approximate position of the surfacebeing ground has been shown in dotted lines. The lip 38 is hererepresented as lying approximately along the element 4| from slightlyabove point C outwardly on the cone to the point where it intersectstheelement.

As seen in Fig. 6, the drill axis makes a, minimum angle ofapproximately six and one-half degrees with axial plane A. In otherwords, if the axis is projected perpendicularly onto plane A, theprojection will form an angle of (S with the drill axis. Surprisingly,this 6 /2 inclination of the drill axis gives a much larger apparentclearance angle, and it isthe apparent clearance angle (measured withreference to point A in Fig. 4) which is customarily made 12 for theso-called 12 clearance. Hence the surface of a drill positioned asdescribed will very closely match the conventional surfaces.

This positioning of the drill does not make the central angle shown inFig. 5 quite as large as is generally desired. Hence it is at presentpreferred to rotate the drill about its axis until this angle is thedesired size. 120-125 is at present preferred for this angle because itgives the best drilling results. i

For very close matching of conventional surfaces, the drills dead centerC should be very carefully positioned along the chosen element 4|.Analysis has indicated that a one-inch drill should have its dead centerC positioned apto which is to be ground, it is sometimes desirabletopreliminarily increase the distance from the vertex so that the drillwill not on account of its odd original shape, strike the far side ofthe cone. I

Although the positioning of the drill described has been chosen forgiving close approximation to given surfaces, it should be recognizedthat some variations in positioning may later be preferred in order tomatch more closely some one part of conventional surfaces or some onepart of a theoretically perfect surface.

Drill holding unit The general natural of the drill holding unit 2| hasalready been described. However, it has a number of special featureswhich warrant further attention.

It will be observed that the cradle ring or carriage 25 is pivoted at 45about "a pin 41 whose axis intersects the axis of chuck 24 and isparallel with, and during grinding coincides with, the chosen elementAll of the cone. Of course the axis of pin 41 could be alined with anyelement of the cone and whatever element it is alined with would becomethe chosen element, the axis of the chuck 24 intersecting the axis ofpin ll. The pin 41 is preferably inclined at an inclination of about 16degrees, thus choosing the element of the cone-which has the sameinclination. The purpose of this is merely to make the axis of the chuckand hence the axis of the drill approximately horizontal as a matter ofconvenience.

It will be observed that the pin l'l is carried by bracket 48 forming anextension of pedestal 49. Secured at the opposite side of pedestal 49 isan arcuate brace 5| which is shaped concentrically about the axis of thepin W. This arcuate brace is seen best in Fig. 8. As seen in Figs. 7 and8, the cradle or carriage 25 is provided with an extension 52 straddlingthe arcuate brace 5| and having a lock screw 53 for locking the cradleat any pivoted position about the pin 41. This adjustment determines theclearance angle of the drill and in each shop will ordinarily be set forthe clearance angle desired by that shop. The brace 5| may be calibratedwith the actual or apparent clearance angles which will result from eachsetting.

The pedestal 49 is slidably carried by. slide plate 56, seen best inFig. 8. The slideway of this plate is parallel to the chosen element ofthe cone and hence parallel to the pin 41. The result is that movementof the pedestal 49 on slide plate 56 adjusts the position of the drillalong the element of the cone without changing anything else. Theposition of the drill may be further adjusted by movement of crank 85. I

The slide plate 56 is in turn slidably carried by the base. 21 with amovement which is axial of the grindstone. The movement of the unittoward the grindstone is limited by stop screw 58 which is adjustable tomake accommodation for variations in thickness of the grindstone H andfor wear of its grinding surface. Of course a lock nut 59 or other suchdevice is provided.

Some means should also be provided for hold ing slide plate 56 againststop screw 58. Any biasing or clamping device could be used. In theillustrated form, it is a handle-operated clamp which is loosened bya'movement away from stone I so that a continuation of this movementwill move the unit away from the stone for removing one drill andinserting another.

Chuck Because this apparatus is adapted to grind a wldevanety of drillizes. it is desirable to have,

atlases a: chuck-which: will? be capable of: holding such a widevariety-of i izes'. Although: the. chuck illustrated in. Figs-.9? andhas'beenx specially devised. for the purposeof'the present invention, itis alsosuitable'fon numerous other purposes and in: fact is highlyadvantageouswhenever a wide variety of sizes or'rodlike membersis to beheld.

The accommodation ofa wide variety of drill sizeswas complicated by twofactors. One is the irregularity in the shaperof the drills, not onlydue to the presence offiutes thereon but also because each flute isprovided along one edge with a rib E! which leaves-clearance between therest of the drill and the ides of the hole which has been drilled; Theother factor which complicates the problem is the vital necessity ofexactly centering all of the drills in order that when it is turned180"degrees:about its axis, the second heel will be ground exactly aswas the first According to the present invention, the chuck requirementshave been solved b using two long sliding jaws which have staggeredinterfitting v -notch'ed teeth 62 and which slide in a V-shaped recessin the chuck. cylinder 63 which supports the jaws-ififi-andpressestherntoward one another along a'substantial portion of their length.

The staggered 'positioning'of the teeth of one jaw with'respect to:those of the other is best seen-in 7'; These teethare preferably longenough' to accommodate whatever maximum size of drill is to behandled;say one and one-half inches. The recesses between the teeth arepreferably deep enough to accommodate the teeth of the opposing jawuntil the jaws have been completely closed-so that they'can' hold a verysmall drilL. Of course this depth; may-be slightly less if, say, nothingsmaller than zone+eighth inch is to be ground with a particular machine.

Of course the jaws may he slid in the chuck cylinder'53 in anyconvenient manner. illustrated forms; each jaw is moved by a pin or keyplate 65 which ispressed in end portions of aslot (ii-in a-bloclz-tfi.To the block 68 in Figs. 9 and 10 is swivelled-a rod (is which isthreaded to a sta-tionaryblock H sothat'turning the handle 12 moves theslid-ing'block- -63-- forwardly and rearwardly which in turn moves thejaws 26 forwardly or rearwardly in their V-shaped recess. The parts areheld in proper position by cover plate It which also serves to brace thesides of the chuck cylinder 63. vOn rearward movement, jaws 26 are drawnapart by interaction between splines 84 and grooves 16.

The movement of block 58 is accomplished slightly diiferently in theother figures, a Worm rack 15 being secured thereto and being moved byan internally threaded ring 11 which is journaled at the end of chuckblock 83.

After the chuck cylinder 53 is slid into the cradle 25, the handle 23 isscrewed into the chuck cylinder 63 for operating the same.

The drill is accurately positioned within the chuck by latching thechuck in the position shown in Fig. 1 and holding the drill lip againstthe corner 18 of gauge 22 while the chuck is tightened. This determinesboth the axial position of the drill and its angular position about itsaxis. For the latter purpose, the sense of touch may easily be reliedupon for getting the lip flat against the underface 19 of the gauge. Itwill be noticed that underface i9 is very narrow at one end thereof. Thereason for this is so that in small drills, it will not strike the otherland than that against which the gauge is being applied. It is notnecessary to provide a stop for the gauge 1.0 in its activepositionrasit willndropxdovm on'th'e 1ip:of the drill.

The depth of cut-is determined by-thumbscrew 8| carried by a bracket82.1 on cradle 25. which limits the forward .positionof chuck-24 as thehandle 231 is pressed down in curved: end portion 3!)"0f 'slot29i-Duringpositioning of the drill in the chuck, thumb screw: 8|'is-turned'to its zero position; after which it isturned to the: depthof grinding desired. Alternatively, the latch 28 'could be madeself-sufficient for holding handle 23 in the zero position and-stopscrew fi l could then be left in a-..de'sired position for depth ofgrind.

The face 19 of gauge 22 determines the final position of the lip ofthe-drill. At: the present time, it is preferred-thatthis*facewill 'lie atan angle which, as viewed iroma direction parallel with theaxis of the'drill, appears to -slope 12 abovethe horizontal upwardly in adirectionaway from the axis ofthe drill. Since the gaugepivots to slightlydifie'rent positions for'difierent siz'esof drill, there will be slightvariations in thisslopa but not enough to-b'e-objectionable. This slopemakes the angle-shown in Fig.- 5' approximately 123 and-the slightvariations due to size of the drill do not carry this beyondsatisfactory limits. Anything'from to125 is believedto be highlysatisfactory and many people believe that the angle can be as large asAlthough this chuck and the cradle'orcarriage and slide arrangementsdescribed: are-especially suitable for the purposes of the presentinvention,

it is recognizedthat inpressing the drill against the internal grindingsurface in accordance with this-inventionza wide variety ofholding-devices may beused, and-"additional adjustments: may be desired.For example, it may be desirable to have an adjustment :for the anglebetween the axis of the drill and the axis of the'pin: 4 1i; Adjustmentsmay also be provided for position:ofth'e-motor. For example, it maybe:found tdbe'desirable to raise and lower the motor, or-move itin'ad-i-. rection perpendicular 'to'the chosen element and tangent tothe cone in order to adjustably-position the lip ofthe drill closertothe chosen element; or slightlytherebelow: to give a-greater increase ofthe clearance angle inwardly in the region close to the cutting lip.

It is quite possible that other positions for the pin 4! and perhapsother inclinations for the sliding action of bracket 49 will be founddesirable. For example, there would be some advantage in shifting thepin 41 to a position approximately coinciding with an extension of thecorner 78 so that change of clearance angle could be made about an axisapproximately coinciding with the lip of the drill, as can be done whenthe drill is in the position shown in Fig. 6.

It is desirable that the grinding surface be kept clean during grinding.This requires special attension because the stone is not self-cleaninginasmuch as the internal angle is such that centrifugal force tends tocause the debris to cling against the rough conical wall instead ofbeing thrown out from the stone. The stone may be kept clean by acontinuous blast of air liberated near the apex. It is preferred to usea vacuum cleaner with a nozzle extending along an element of the coneand reaching close to the apex by passing between the drill and the conesurface. The wear on the stones is not excessive. Usually, various sizesof drills will be ground and their wear will be spread over differentareas of the cone surface. Slight departures of a true cone are not Initerially objectionable and do not cause any noticeable difference inthe ground surface. As often as may be desired, however, the stone maybedressed to an accurate conical surface by inserting a diamond-tippeddressing tool in the chuck, adjusting the holding unit for the properdepth of cut in the stone and turning the handle 85 rapidly as the motordrives the stone to move the diamond tip along the length of an elementof the new surface. It will be observed that this jaccurate parallelismbetween the chosen element :of the cone and the movement of the tips ofthe drills held in the chuck so that turning the bandle 85 to positionthe chuck properly for difierent drill sizes will merely move the drillalong this element without altering the depth of grind :for which thethumb screw 8| has been set.

Analysis has indicated that departures of shape of the surfaces groundin the foregoing manner are extremely slight as compared with atheoretical surface which wouldbe generated by movement of the cuttinglip along a helix of the desired pitch. For example with the grindingposition shown in Fig. 6, calculations have indicated that the maximumdeparture from the theoretical surf-ace is only .03 inch while thedeparture along the cutting edge is even less. Of course the ultimatetest is in satisfactory drilling and that test has been met by theproducts of this jticing' this method in a simple manner and withadequate adjustability, but with extreme uniformity for any adjustment.I claim as my invention:

1. Drill sharpening apparatus including a grindstone having an internalconical surface centered about its axis and a drill holder including acarriage, a chuck slidably and' rotatably mounted in the carriage, meansfor initially correctly positioning the drill in the chuck coaxiallytherewith, means for swinging the chuck about the common axis todifferent positions to place first one heel of the drill and then theother in grinding position against the said internal surface of thegrindstone with the center of the drill point displaced from the axis ofthe stone, and for advancing the chuck axially to a predetermined finalposition, and means for mounting the carriage tiltably about an axisapproximately coinciding with the cutting edge of the drill in theposition to which it is finally ground whereby the clearance anglesbehind said cutting edge may-be adjusted while leaving other factorsunchanged.

2. Drill sharpening apparatus including a grindstone having an internalconical surface centered about its axis and a drill holder including acarriage, a chuck slidably and rotatably mounted in the carriage, meansfor initially correctly positioning the drill in the chuck coaxiallytherewith, means for swinging the chuck about the common axis todifferent positions to place first one heel of the drill and then theother in grinding position against the said internal surface of thegrindstone with the center of the drill point displaced from the axis ofthe stone, and for advancing the chuck axially to a predetermined finalposition, and means for mounting the carriage tiltably about an axisapproximately'parallel with the cutting edge'of the drill in theposition to which it is finally ground whereby the clearance anglesbehind said cutting edge may be adjusted.

MELVIN DOUGLAS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 282,364 Pickering July 31, 18831,994,975 Williams Mar. 19, 1935 2,017,532 Elter et a1 Oct. 15, 19352,068,915 Hardin Jan. 26, 1937 2,245,858 Hornberger June 17, 19412,305,230 Allen Dec. 15, 1942

